The present invention relates to packaging of gallium arsenide (GaAs) integrated circuits, and, more particularly, to a hydrogen getter for chemically binding up hydrogen evolved from packaging materials.
Gallium arsenide (GaAs) integrated circuits which are hermetically packaged will suffer from reduced performance and reliability if the hydrogen that is evolved from the packaging materials is allowed to diffuse into the GaAs devices. By xe2x80x9cpackaging materialsxe2x80x9d is meant base materials, such as iron-nickel alloys (housing, cover, etc.), platings, such as nickel and gold plating (both electroless and electrolytic, used for conductive traces and surfaces as well as for corrosion-resistant surface treatments), and organic materials, such as epoxy for sealing or die attachment. Hydrogen concentrations as low as 500 ppm have been demonstrated to decrease the mean time to failure. A prior art solution to this problem of hydrogen poisoning is to insert a material into the package which chemically binds up the hydrogen. This material is typically referred to as a hydrogen getter.
In many critical applications, reducing the size, weight, and cost of the hydrogen getter is crucial to improving the product line. In some applications, mainly those involving organic electronic packaging material, some type of metallization is required to provide a ground path/EMI (electro-magnetic interference) shield, due to the relatively high resistivity of the organic material.
Alpha Metals (Jersey City, N.J.) produces a hydrogen getter which incorporates particles of a gettering material into a silicone matrix sheet. There are several disadvantages to this getter: (1) in order to provide enough hydrogen absorption capacity in a package, a significant volume of silicone getter must be used; (2) Residual Gas Analysis (RGA), used to determine internal contaminants within a hermetic package, can return false results related to moisture content when the silicone product is used; (3) silicone oil migration within the package has been observed, which makes re-work of the assembly impossible, due to the inability to re-solder joints wetted with the silicone oil; (4) there is added cost and complexity associated with having to bond the silicone getter sheet to some available surface; and (5) the silicone getter must first be vacuum baked at 150xc2x0 C. for greater than 16 hours before it can be inserted into the package. This adds to the manufacturing time and cost.
Several years ago, titanium foils coated with vacuum-deposited palladium were investigated as potential hydrogen getters by Hughes Aircraft. These getters did not provide good reliable gettering capability, most likely due to the oxide layer present on the titanium foil. A titanium foil would also have the added cost of welding to the package lid.
Packaging density has increased four fold in the last six years of transmit/receive (T/R) module development; such modules employ interconnect frames. Traditional 2-D packaging solutions typically exhibit large planar areas, such as the cover, that are suitable for mounting commercially-available hydrogen getter technology. However, the cover in a 3-D package is now an electrically functional component and not suitable for location of a hydrogen-gettering material.
Thus, there is a need for a hydrogen getter that will provide significant improvement in the size, weight, cost, flexibility, and ease of insertion while at the same time achieving excellent hydrogen gettering capability both in terms of the speed at which the getter absorbs hydrogen and the overall amount of hydrogen that can be absorbed. In addition, a means of designing the film to meet the service life of the product is also necessary, not only from a total absorption but from a film integrity perspective. The reaction degradation of the film or gettering media cannot result in conductive particle generation, which would present functional risk to the packaged electronics. There is also a need for a ground/EMI shield where the electronics package material comprises an organic polymer.
In accordance with a first aspect of the present invention, a combination is provided of (a) a thin film hydrogen getter for gettering hydrogen evolved from packaging materials employed in a device comprising hermetically-sealed GaAs integrated circuitry employing at least one interconnect frame and (b) an EMI shield for shielding internal signals. By xe2x80x9cthin filmxe2x80x9d is meant herein a metal film that is vacuum-deposited, such as by sputtering or evaporation.
The thin film getter and EMI shield of the present invention comprises a multilayer metal film that is vacuum-deposited. The multilayer film comprises:
(a) a layer of an electrically conductive metal for providing electro-magnetic interference shielding, formed on surfaces of the interconnect frame;
(b) a layer of titanium for absorbing and chemically binding up the hydrogen, formed on the layer of electrically conductive metal; and
(c) a layer of palladium for preventing oxidation of the titanium, but permeable to the hydrogen, formed on the layer of titanium.
A method of fabricating the combination is also provided. The method comprises:
(a) forming the layer of the electrically conductive metal on surfaces of the interconnect frame;
(b) vacuum-depositing the layer of titanium on the layer of electrically conductive metal; and
(c) vacuum-depositing the layer of palladium on the layer of titanium.
Both the titanium and the palladium are deposited during the same coating process (vacuum deposition) run, thereby preventing the titanium from being oxidized. The palladium continues to prevent the titanium from being oxidized once the getter is exposed to the atmosphere. However, hydrogen is easily able to diffuse through the palladium into the titanium where it is chemically bound up, since palladium is highly permeable to hydrogen. The present inventors have demonstrated high hydrogen absorption rates and hydrogen capacities for thin film getters deposited onto plastic test parts.
Finally, in accordance with another aspect of the present invention, a hydrogen getter is provided for gettering hydrogen evolved from packaging materials employed in a transmit/receive module configured to transmit and receive electromagnetic radiation over a predetermined portion of the electromagnetic spectrum. The transmit/receive module comprises a least one frame component formed as a single piece from a synthetic resin dielectric material. The frame component is configured to support a plurality of electrical connectors. The hydrogen getter comprises a thin film coating on at least one surface of the frame component. The thin film coating comprises (a) the layer of titanium and (b) the layer of palladium, as described above.
The thin film getter has several advantages: (1) Since the thin film getter has a hydrogen capacity per unit volume which is 25 times higher than the silicone getter, then the thin film getter will occupy an extremely small volume; (2) It is flexible in its application, since any convenient substrate that is going into the hermetic package can be coated with the thin film getter; (3) There is no cost or time associated with welding or bonding the thin film getter into the package as there is with foil or silicone getters; (4) The thin film getters don""t require a lengthy vacuum bakeout prior to insertion; a vacuum bakeout of 85xc2x0 C. for 2 to 3 hours is sufficient; and (5) There is no silicone in the thin film getter, which prevents contamination.
In addition to the advantages that the thin film getter provides over the other getters, as described in greater detail below, the thin film getter is also able to absorb hydrogen effectively at temperatures as low as 0xc2x0 C.
The ground/EMI shield can be used to enhance circuit performance. The ground/EMI shield film component allows organic materials to be used within microwave modules, thereby providing cost benefits through reduced weight. The conductivity of the film allows its use in any type of dielectric package, thereby providing EMI and hydrogen protection for GaAs circuitry.